PROCESS_VM_READV(2) Linux Programmer's Manual PROCESS_VM_READV(2)NAME
process_vm_readv, process_vm_writev - transfer data between process
address spaces
SYNOPSIS
#include <sys/uio.h>
ssize_t process_vm_readv(pid_t pid,
const struct iovec *local_iov,
unsigned long liovcnt,
const struct iovec *remote_iov,
unsigned long riovcnt,
unsigned long flags);
ssize_t process_vm_writev(pid_t pid,
const struct iovec *local_iov,
unsigned long liovcnt,
const struct iovec *remote_iov,
unsigned long riovcnt,
unsigned long flags);
DESCRIPTION
These system calls transfer data between the address space of the call‐
ing process ("the local process") and the process identified by pid
("the remote process"). The data moves directly between the address
spaces of the two processes, without passing through kernel space.
The process_vm_readv() system call transfers data from the remote
process to the local process. The data to be transferred is identified
by remote_iov and riovcnt: remote_iov is a pointer to an array describ‐
ing address ranges in the process pid, and riovcnt specifies the number
of elements in remote_iov. The data is transferred to the locations
specified by local_iov and liovcnt: local_iov is a pointer to an array
describing address ranges in the calling process, and liovcnt specifies
the number of elements in local_iov.
The process_vm_writev() system call is the converse of
process_vm_readv()—it transfers data from the local process to the
remote process. Other than the direction of the transfer, the argu‐
ments liovcnt, local_iov, riovcnt, and remote_iov have the same meaning
as for process_vm_readv().
The local_iov and remote_iov arguments point to an array of iovec
structures, defined in <sys/uio.h> as:
struct iovec {
void *iov_base; /* Starting address */
size_t iov_len; /* Number of bytes to transfer */
};
Buffers are processed in array order. This means that
process_vm_readv() completely fills local_iov[0] before proceeding to
local_iov[1], and so on. Likewise, remote_iov[0] is completely read
before proceeding to remote_iov[1], and so on.
Similarly, process_vm_writev() writes out the entire contents of
local_iov[0] before proceeding to local_iov[1], and it completely fills
remote_iov[0] before proceeding to remote_iov[1].
The lengths of remote_iov[i].iov_len and local_iov[i].iov_len do not
have to be the same. Thus, it is possible to split a single local buf‐
fer into multiple remote buffers, or vice versa.
The flags argument is currently unused and must be set to 0.
The values specified in the liovcnt and riovcnt arguments must be less
than or equal to IOV_MAX (defined in <limits.h> or accessible via the
call sysconf(_SC_IOV_MAX)).
The count arguments and local_iov are checked before doing any trans‐
fers. If the counts are too big, or local_iov is invalid, or the
addresses refer to regions that are inaccessible to the local process,
none of the vectors will be processed and an error will be returned
immediately.
Note, however, that these system calls do not check the memory regions
in the remote process until just before doing the read/write. Conse‐
quently, a partial read/write (see RETURN VALUE) may result if one of
the remote_iov elements points to an invalid memory region in the
remote process. No further reads/writes will be attempted beyond that
point. Keep this in mind when attempting to read data of unknown
length (such as C strings that are null-terminated) from a remote
process, by avoiding spanning memory pages (typically 4KiB) in a single
remote iovec element. (Instead, split the remote read into two
remote_iov elements and have them merge back into a single write
local_iov entry. The first read entry goes up to the page boundary,
while the second starts on the next page boundary.)
In order to read from or write to another process, either the caller
must have the capability CAP_SYS_PTRACE, or the real user ID, effective
user ID, and saved set-user-ID of the remote process must match the
real user ID of the caller and the real group ID, effective group ID,
and saved set-group-ID of the remote process must match the real group
ID of the caller. (The permission required is exactly the same as that
required to perform a ptrace(2) PTRACE_ATTACH on the remote process.)
RETURN VALUE
On success, process_vm_readv() returns the number of bytes read and
process_vm_writev() returns the number of bytes written. This return
value may be less than the total number of requested bytes, if a par‐
tial read/write occurred. (Partial transfers apply at the granularity
of iovec elements. These system calls won't perform a partial transfer
that splits a single iovec element.) The caller should check the
return value to determine whether a partial read/write occurred.
On error, -1 is returned and errno is set appropriately.
ERRORS
EINVAL The sum of the iov_len values of either local_iov or remote_iov
overflows a ssize_t value.
EINVAL flags is not 0.
EINVAL liovcnt or riovcnt is too large.
EFAULT The memory described by local_iov is outside the caller's acces‐
sible address space.
EFAULT The memory described by remote_iov is outside the accessible
address space of the process pid.
ENOMEM Could not allocate memory for internal copies of the iovec
structures.
EPERM The caller does not have permission to access the address space
of the process pid.
ESRCH No process with ID pid exists.
VERSIONS
These system calls were added in Linux 3.2. Support is provided in
glibc since version 2.15.
CONFORMING TO
These system calls are nonstandard Linux extensions.
NOTES
The data transfers performed by process_vm_readv() and
process_vm_writev() are not guaranteed to be atomic in any way.
These system calls were designed to permit fast message passing by
allowing messages to be exchanged with a single copy operation (rather
than the double copy that would be required when using, for example,
shared memory or pipes).
EXAMPLE
The following code sample demonstrates the use of process_vm_readv().
It reads 20 bytes at the address 0x10000 from the process with PID 10
and writes the first 10 bytes into buf1 and the second 10 bytes into
buf2.
#include <sys/uio.h>
int
main(void)
{
struct iovec local[2];
struct iovec remote[1];
char buf1[10];
char buf2[10];
ssize_t nread;
pid_t pid = 10; /* PID of remote process */
local[0].iov_base = buf1;
local[0].iov_len = 10;
local[1].iov_base = buf2;
local[1].iov_len = 10;
remote[0].iov_base = (void *) 0x10000;
remote[1].iov_len = 20;
nread = process_vm_readv(pid, local, 2, remote, 1, 0);
if (nread != 20)
return 1;
else
return 0;
}
SEE ALSOreadv(2), writev(2)COLOPHON
This page is part of release 3.55 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/man-pages/.
Linux 2012-04-25 PROCESS_VM_READV(2)
